The invention relates to a solar panel comprising solar cells affixed to one or more sheet elements and a frame for supporting said sheet elements, wherein the frame comprises one or more pretensioned strings for attaching said sheet elements to the frame, the sheet elements being attached to said strings.
A reduction in solar cell mass can be achieved with a known process to make solar cells with a thin film process. According to this known process the solar cell material is manufactured in the form of thin-films, which comprise multiple layers of e.g. vapour-deposited material. These thin-films are relative delicate and can not be used as structural elements. Therefore, according to the prior art, these thin-films are attached to a backing material such as a sheet of metal or plastic foil.
This backing material is, in order to provide sufficient constructional rigidity, relatively heavy. Especially for spacecraft applications such a heavy backing material should be avoided. Besides, usually the sheet or foil needs to be stressed to increase rigidity of the structure to a sufficient extent. At the interconnects between the cells, stresses will occur. These stresses between the solar cells may lead to difficulties.
U.S. Pat. No. 4,293,731 describes a solar cell generator especially for spacecrafts, consisting of a rectangular frame design with solar cells bonded on a lightweight sheet. Tensioning elements are operatively interposed between the frame structure and the panel sheets. Herein the solar cells are bonded with a small gap between adjacent solar cells and means are interconnected to each other to collect the electrical current and to transfer this electrical current by wires to the spacecraft. Stay wires extend along lateral edges of the sheets. These wires are also secured to the tensioning means.
The thin light-weight sheets of solar cells have certain maximum dimensions limited in the production process. As a result, the frame structure of the solar panel has to be dense, which leads to a high weight.
EP 218 858 discloses a solar panel which comprises solar energy collectors. These collectors are composed of strip-formed mirrors that have solar cells on their back surfaces. The collectors are mounted on supporting strings that extend between two transverse beams of a central mast. Light impinges on the mirrors, which reflect it towards solar cells of adjacent collectors.
The risk of failure of this solar panel is relatively high. During launch, the solar panel is in a folded position. Once in orbit, the solar panels are unfolded, wherein the strings have to be unrolled. The unrolling of the strings involves a complex mechanism that does not comply with the high reliability standards of the space industry.
Another solar panel assembly is known from EP 926 068. This solar panel assembly comprises rectangular panels each carrying solar cells. The panels are pivotally interconnected. Tensioning wires are used to bend the panels in order to increase the structural stiffness of the extended solar panel assembly. The tensioning wires may extend through openings in the panels.
The tensioning wires are operated after unfolding the solar panel assembly from a folded position, which may lead to difficulties. Moreover, the solar cells are not thin film sheets. The panels are therefore not supported in a frame.
The object of the present invention is to provide a solar panel according to the introduction, wherein the structural characteristics thereof are improved.
According to the present invention that object is achieved in that the sheet elements are composed of flexible material and are attached to the strings in a corrugated shape
According to the invention it is possible that the sheet elements are provided with at least one row of apertures for weaving a string through successive apertures of said row.
By means of the above-mentioned measures, it is achieved that instead of using a relatively heavy backing material, the sheet elements are attached to a frame for supporting the sheet elements by means of the mentioned pretensioned strings. Since the sheet elements are attached to these strings in a corrugated shape, in the sheet elements a certain amount of elastic bending energy will be stored. The bending lines which are present in the sheet elements provide rigidity for the sheet elements. The combination of using strings and bending the respective sheet elements is sufficient for creating the required rigidity of the solar panel.
Moreover, compared to a known solar panel wherein the solar cells are mounted on a separate pretensioned sheet, mounting solar cells on strings according to the invention strongly or fully reduces stresses between the solar cells at their interconnects, irrespective of pretensioning the strings.
According to a preferred embodiment the solar cells comprise an electrical conductive backing material, which forms said sheet elements.
According to another embodiment the solar cells comprise an electrical conductive backing material, which is mounted on said sheet elements.
If the solar cells with their backing material are bonded to sheet elements, stresses may occur in the solar cell interconnects due to differences in thermal expansion of said solar cells with their backing material on the one hand and the material of said sheet elements on the other hand. Omitting a separate sheet element, i.e. said backing material serves as sheet elements, essentially eliminates such deviating thermal expansion.
According to the present invention it is possible that a series of successive sheet elements is attached to each string or each set of strings.
According to known techniques the dimensions of the individual sheet elements which can be manufactured by means of vapour-deposited materials are limited. In order to be able to provide the solar panel with a sufficient surface area, a series of individual sheet elements has to be provided. Respective sheet elements are attached to the strings in order that the sheet elements themselves abut to adjacent sheet elements.
According to the present invention it is possible that elastic bending energy is stored in each of the sheet elements, wherein the sheet elements are attached to the strings in order to cause the tips of each of the sheet elements to be urged towards the tips of adjacent sheet elements as a result of the bending energy stored in each of the sheet elements.
Because of the fact that the respective elements are urged towards adjacent elements a contact area between said elements is created. This contact area provides the electrical contact between adjacent sheet elements.
According to an alternative embodiment it is possible that the sheet panels are connected to adjacent sheet elements by means of electrical conductive adhesive.
According to a further aspect of the present invention it is preferred that each of the strings is provided with a stop element at respective ends of the string, in order to enclose the sheet elements attached to the string between said stop elements.
The stop elements are at least provided at the respective ends of the strings. The presence of the stop elements provide a very simple and cheap fixing means for fixing the respective bended sheet elements between two stop elements.
According to the present invention it is possible that the stop element is composed of electrical conductive material.
Such an electrical conductive stop element can be used in order to connect an electric wire in order to guide an electric current created in the solar panel into an electric circuit.
According to the invention it is possible that the strings are composed of electrical non-conductive material.
Because of this measure it is avoided that current may leak towards the structural frame of the solar panel. In other words, when the strings themselves are electrical nonconductive no current can be guided through the strings towards a frame part.
It is preferred that each of the strings is provided with at least one spring element at a respective end thereof in order to stretch the respective string.
The spring element will be used in order to stretch respective strings which are present in the solar panel string. When the solar panel is used the strings in the solar panel may lengthen. This may reduce the pretension which is present in the strings. Because of the presence of the spring elements the tension in the strings can be maintained at a required level.
According to a further aspect the invention relates to a solar panel assembly, comprising two or more solar panels according to the present invention. Moreover, the invention relates to a spacecraft which is provided with the solar panel according to the present invention.